Sudan's first multi-party elections in 24 years are going into their second day amid reports of confusion and disarray in many regions.
While the process generally went well in the capital Khartoum, voters faced obstacles in several states from the Red Sea in the north to the far south.
The dominant party in the south is calling for a four-day extension.
The presidential, parliamentary and state assembly polls are part of the deal that ended Sudan's civil war.
BBC Africa editor Martin Plaut, in Khartoum, says Sudan has seen little peace since independence in 1956, and to hold an election as complex as this one, in a country so underdeveloped, was always going to be a tall order.
'A good feeling'
It is widely expected that the country's two most influential men, President Omar al-Bashir, and Silva Kiir, who leads largely autonomous Southern Sudan, will retain their positions.
Mr Bashir is seeking a democratic mandate since being indicted by the International Criminal Court for alleged war crimes in Darfur but a boycott of the poll by his two main challengers means his mandate is likely to be reduced.
Mr Kiir, who is standing unopposed, was forced to wait for his polling station to open in the southern capital Juba but he said afterwards that he had a "good feeling" about the country's political future.
"I have never voted in my life," he said. "This is my first time to vote and it is a good feeling that Sudan is going back to democracy."
Voting in parts of Khartoum was held up by delays in getting ballots to polling stations, ballot mix-ups and names missing from the electoral roll, Reuters news agency reports.
In the south, many polling stations opened late and many voters, including senior officials, could not find their names on voter rolls.
'Quiet in Darfur'
The elections are also complicated by the ongoing low-level civil war in Darfur, where some three million people are living in refugee camps.
The BBC's Mohamed Khalid, in the Darfur city of Fasher, says the turnout was surprisingly high there amid tight security, but there were no reports of any rebel attacks.
The north-south civil war ended in 2005, with a deal for the Sudan People's Liberation Movement (SPLM) to share power with Mr Bashir's National Congress Party nationally, while running affairs in the south on its own.
For many in Southern Sudan, these elections are a prelude to a referendum next January on possible independence.
President Bashir has said he will accept the referendum result, even if it favours independence for the south.
However, the country's oil fields lie along the north-south border and some fear that an independence bid could lead to renewed conflict.